The invention relates generally to a continuous wave (CW) device for measuring frequency shifts in the peak of a mechanical resonance in a body due to changes in the physical characteristics of the body and more specifically concerns a CW ultrasonic device for measuring strain in a bolt.
Although there are many methods available for measuring strain in bolts, the torque wrench is the general accepted technique and justly so-for most applications. However, for critical assemblies, the accuracy of a torque wrench may lead to improperly stressed bolts due to friction between the threads as well as between the nut and the work. The "calibration" of torque wrenches assumes that 40 percent of the usable applied force is dissipated in friction but in fact, this number varies greatly with configuration, roughness, lubrication, etc. Because of the calibration uncertainty, components bolted with torque wrenches must be over-designed leading to increased cost and weight or must suffer a reduced reliability. An excellent method for measuring bolt strain is the strain gage bolt, which uses a standard resistance strain gage mounted to the body of the bolt. This approach results in a highly accurate bolt strain monitor at a cost of several hundreds of dollars per bolt but measures only the local strain under the gage. Another technique used if both sides of the bolt are accessible, is a simple caliper to directly measure bolt elongation. However, in many instances this is not possible and a more sophisticated approach is required.
An ultrasonic technique using pulse echo is available which can resolve stress produced steel bolt elongation for one echo on the order of .+-.10.sup.-2 mm. Although this is excellent length resolution, a 0.1 percent elongation of a 100-mm bolt, based on this resolution corresponds to a potential strain accuracy of only .+-.10 percent with less strain having correspondingly poorer accuracy. In addition, pulse-echo systems must utilize fast risetime pulses to achieve such accuracy which adds to the cost and complexity of the technique. Although this resolution may be improved with higher echo numbers, this places a constraint on bolt parallelism.
It is therefore the primary object of this invention to provide a CW ultrasonic device for measuring the strain in a bolt that is simple in operation, that has high accuracy and that has a potential low cost.
Another object of this invention is to provide a CW ultrasonic device for measuring frequency shifts in the peak of a mechanical resonance of a body due to changes in the physical or electrical characteristics of the body.
A further object of this invention is to provide an improved reflection type CW ultrasonic transducer.
Other objects and advantages of this invention will become apparent in the following specification and drawings.